Ultrasonic inspection is commonly used to detect flaws, such as surface flaws (e.g., cracks), internal flaws (e.g., voids or inclusions of foreign material), and other defects. It is also used to measure wall thickness in tubes and pipes as well as bar diameters. In what is known as the pulse-echo method for testing, the same transducer serves both as a transmitter and a receiver of ultrasonic beams or waves used to detect such flaws and take such measurements.
When testing using the pulse-echo method, a transducer produces a pressure wave referred to as an ultrasonic pulse in response to an electrical pulse. The pressure wave travels through a coupling medium between the transducer and the tested object. Once the ultrasonic pulse reaches an interface between the coupling medium and the tested object, a portion of the pulse enters the object whereas another portion is reflected back to the transducer (i.e., a partial reflection and transmission occur). The initially reflected pulse is known as a frontwall echo. The portion of the pulse that enters the object continues until the back wall, where another partial reflection and transmission occur. This partial reflection is known as the backwall echo. If there is an internal flaw in the tested object for instance, a portion of the ultrasonic pulse is also reflected back to the transducer at the flaw. The flaw can be located knowing the elapsed time between the different reflections.
For automatic flaw testing, a gate is placed between the frontwall and backwall echoes. Any pulse within the gate area may be peak detected, producing an analog output that can be recorded and that represents a flaw in the tested object. In addition, thickness measurements are made possible knowing the time difference between the backwall and frontwall echo pulses as well as the velocity of the ultrasonic wave as it travels through the medium of the tested object.
The most widely used pulse-echo process for non-destructive testing of objects such as tubes and bars is performed by using ultrasonic rotary testers. Ultrasonic transducers are mounted on a rotary testing unit of such testers, while the tube or bar to be tested is moved freely through the tester. Rotating the transducers in the tester around the tube as opposed to rotating the tube as it is moved through the tester eliminates the need for heavy machinery and high power in the case of testing large and long tubes and bars. The space between the object and transducers is generally filled with water in order to provide coupling for the ultrasonic beam. The electrical signals from the ultrasonic inspection instrument are connected to the rotating transducers by rotary capacitors. In order to detect various kinds of surface and internal flaws and to provide thickness measures, several transducers may be mounted on the tester, each being oriented to perform a specific function.
For instance, in a longitudinal wave inspection arrangement, a transducer is typically oriented so that the ultrasonic beam is perpendicular to the surface of the tested object. In such a case, the angle of incidence is 90 degrees. The resulting longitudinal waves travel along a path that is aligned with the radial axis of the tested object and are therefore suitable for taking thickness or diameter measurements and detecting certain inner flaws.
When the angle of incidence is not 90 degrees, refraction occurs and the ultrasonic beam splits into two parts in a solid material: a longitudinal wave beam and a shear wave beam. In longitudinal waves, particle motion is parallel to the direction of wave propagation. In shear waves, however, particle motion is perpendicular to the direction of wave propagation. The refraction angle of the longitudinal wave beam is greater than that of the shear wave beam. Accordingly, when the angle of incidence exceeds a particular value, the longitudinal wave beam ceases to exist and only the shear wave beam remains. This angle is called the first critical angle. Shear waves can be used to detect both surface and internal flaws.
For flaw detection of surface and internal flaws in tubes and bars, shear wave testing is commonly used. FIG. 1 illustrates a setup for performing one type of such a test on a tube. To improve the detectability of irregularly shaped flaws, shear waves are generated in both clockwise and counter-clockwise directions simultaneously using two offset transducers 110. Each incident beam 120 of transducers 110 is maintained within the same plane of a cross section that is perpendicular to longitudinal axis 150 of tube 130, while offsetting it from radial axis 180. The magnitude of offset is proportional to the diameter of the tube.
Under the setup of FIG. 1, beams 122 and 144 travel generally clockwise and counter-clockwise, respectively, in the plane of cross section, bouncing between the outer and inner surfaces of tube 130 until a flaw is detected and beam 120 is partially reflected back to transducer 110. As shown in FIG. 1, the beam traveling clockwise, beam 122, is reflected back from an inner diameter crack 160, while the beam traveling counter-clockwise, beam 144, is reflected back from an outer diameter crack 170.
Such an arrangement, whereby the transducer is offset while its beam remains within the same plane of the tube's cross section, is suitable for detecting longitudinal flaws, i.e., flaws that are generally parallel to the tested object's longitudinal axis. However, in order to detect transverse flaws (i.e., flaws that are generally perpendicular to the tube's longitudinal axis), another arrangement is more appropriate. More specifically, the transducer is preferably angled in a plane containing the tube's longitudinal axis without offsetting the transducer from its position when it performs longitudinal wave testing. FIG. 2 illustrates a setup for performing such a test on a tube.
In FIG. 2, transducer 210 is angled in a plane containing longitudinal axis 150 and radial axis 180 of tube 130 without offsetting transducer 210 from its position when it performs longitudinal wave testing. Incident beam 120 of transducer 210 is maintained within the same plane of radial axis 180 and longitudinal axis 150 of tube 130 without creating the offset depicted in FIG. 1. Beam 222 of transducer 210 travels generally along the length of tube 130, in the plane containing radial axis 180 and longitudinal axis 150, bouncing between the outer and inner surfaces of tube 130 until a flaw is detected and beam 120 is partially reflected back to transducer 110. Beam 222 is partially reflected back from transverse crack 260 and beam 220 is partially reflected back to transducer 210. Transducer 210 can be oriented for forward-, or reverse-looking shear wave testing.
Referring to both FIGS. 1 and 2, tube 130 can be scanned if a set of transducers is rotated around longitudinal axis 150 while tube 130 is freely moved along longitudinal axis 150. To allow for thickness measurement and to ensure full flaw detection, several transducers are mounted in the rotary tester. Transducers can be oriented generally for longitudinal wave testing, while other transducers can be oriented for clockwise and counter-clockwise shear wave testing as shown in FIG. 1 and yet other transducers can be oriented for forward-, and reverse-looking shear wave testing as shown in FIG. 2. In this manner, five channels are required so that each transducer can be individually driven to achieve full-volume testing that measures thickness and detects internal flaws as well as surface flaws.
The above discussion outlines how shear waves can be used to detect internal or surface flaws. Offsetting a transducer without angling it, as shown in FIG. 1, can be used to detect longitudinal flaws, but will very likely miss transverse flaws. On the other hand, angling a transducer without offsetting it, as shown in FIG. 2, can be used to detect transverse flaws but will very likely miss longitudinal flaws. The orientations of longitudinal and transverse flaws may vary +/−5 degrees and still be detected by either offsetting or angling the transducer. However, such shear wave testing will likely miss naturally occurring flaws which are more commonly oriented at some angle such that they are neither longitudinal nor transverse given that the larger portions of the beam will likely not be reflected back to the transducer when bouncing off these flaws. Such flaws may be referred to as oblique flaws.
In view of the foregoing, it would be desirable to provide an ultrasonic transducer arrangement for detecting oblique flaws using pulse-echo testing.